| Energy is the foundation for social development.The severe energy issue urgently requires energy conservation,emission reduction and utilization of renewable.As one of the renewable energy sources,solar energy is widely distributed,rich,clean and pollution-free,inexhaustible and so on.In addition,China is rich in industrial waste heat.Efficient utilization of industrial waste heat not only save energy,reduce costs,but also reduce environmental pollution.However,industrial waste heat and solar energy have some shortcomings,such as distribution,intermittency and instability,which make it difficult to recycle.Phase change energy storage technology is able to store discontinuous and unstable heat,alleviate the contradiction between energy supply and demand,which makes it is one of the effective ways to save energy and reduce emissions.However,in order to realize effective heat storage,phase change materials and heat storage devices with appropriate heat storage properties are needed.In this paper,combining experiments and numerical simulations,we studied the structural optimization of plate-type phase change units(PPCU)and plate-type phase change heat exchangers(PPCHE).The main contents of this study are as follows.Firstly,the phase change heat transfer process within the PPCU is explored experimentally.Taking a closed rectangular space as the experimental model and paraffin as the phase change material(PCM),the melting and heat transfer process of PCM within the PPCUs with different aspect ratios and different thicknesses are mainly studied under the condition of the same mass of the PCM.The main conclusions are as follows:(1)Thermocouples accelerate the melting of paraffin.(2)Natural convection in the melting process of PCM is observed accelerates the melting process of paraffin.(3)Under the influence of suspension force and heat exchange area,the PCM within PPCU with aspect ratio of 3:1 is melted the fastest,while the PCM within PPCU with aspect ratio of 2:3 is slowest melted.Secondly,in order to further explore the natural convection during the melting process of the PCM.Basis on the experimental results,a three-dimensional model of PPCU with a aspect ratio of 3:1 and a thickness of 20 mm was constructed.FLUENT software was used to simulate whether there is natural convection during the PCM melting process,compared with the experimental results.The simulation results show that:(1)the simulation with natural convection is in good agreement with the experimental results,and the error is 2.28%;(2)Natural convection mainly occurs in the vertical space between the phase change interface and the side wall.The liquid paraffin near the phase change interface flows downward,while the liquid paraffin near the wall flows upward,forming a circular flow.(3)Natural convection accelerates the melting of PCM.The melting time in this model is shortened by 15%.Finally,a model of PPCHE was designed,and the influence of geometric parameters on the performance of the PPCHE was studied.By controlling the volume of the heat exchanger and the mass of the PCM,five PPCHEs models with different number of PPCUs were built.Numerical simulation and were carried out optimize the inlet temperature and inlet velocity of the optimized structure.The results show that:(1)Increasing the number of PPCUs increases the heat transfer rate,but at the same time increases the pressure drop.When the number of PPCUs is 5,the heat transfer efficiency is the best;(2)The boundary fluid velocity of the PPCHE is proportional to the energy storage rate and inversely proportional to the pressure drop;(3)The heat transfer rate of PPCHE is proportional to temperature,but the increase of efficiency decreases with the increase of boundary velocity or temperature.(4)In this study,the best condition of energy storage efficiency is v=0.5 m/s and N=5.In practical application,it is necessary to focus on the design of the internal structure of the heat exchanger when the flow rate is low. |
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